Method of forming coating films

ABSTRACT

The invention provides a method of forming a coating film by forming in sequence a pigmented base coat and a clear top coat on a substrate followed by finishing by the two-coat one-bake technique, the method being characterized by using, as a coating composition for pigmented base coat formation, a composition comprising, as essential components thereof, 
     (1) an OH-containing resin, 
     (2) an amino resin, 
     (3) a polyorganosiloxane 
     (4) a flaky metal powder and/or a mica powder, and 
     ( 5 ) an organic solvent, 
     and using, as a coating composition for clear top coat formation, a composition comprising, as essential components thereof, 
     (1) an OH-containing base resin which further contains at least one group selected from the class consisting of a silanol group and a hydrolyzable group bound directly to a silicon atom, 
     ( 2 ) an amino resin, and 
     ( 3 ) an organic solvent.

The present invention relates to a novel method of forming coating filmsand, more particularly, to a method of forming a coating film excellentin finished appearance.

Top coats, particularly those on automotive outer body, are required tobe improved almost constantly in finished appearance and coating filmperformance characteristics. Furthermore, the advent of low-temperaturecurable coating films is eagerly awaited for reducing energy costs andfor insuring that an automotive outer body consisting of steel andplastic materials can be integrally coated with one and the same coatingcomposition.

Coating compositions based on thermosetting acrylic resin/melamineresin, those based on thermosetting polyester resin/melamine resin andthe like are now mainly used as coating compositions for top coat in theautomobile industry. In terms of the degree of levelness and smoothness,these compositions have almost reached their limit, while there is stillmuch room for improvement in these and other film performancecharacteristics. Another problem is that their low-temperaturecurability is not fully satisfactory. Furthermore, such problems asebullition of coating films due to condensation byproducts (formalinetc.) generated in the curing step and environmental pollution remain tobe solved.

The inventors of the present invention previously proposed an automotivecoating composition excellent in low-temperature curability in aJapanese Unexamined patent publication No. 160879/1990. This compositionis a silicone-based composition comprising a copolymer obtained bypolymerizing a silanol- and/or alkoxysilane group-containingpolysiloxane macromonomer and an oxirane-containing vinyl monomer, and ametal chelate compound. However, this composition has a drawback in thatwhen it is used as both a metallic base coat composition (containing analuminum flake pigment) and a clear top coat composition in a two-coatone-bake system, the metallic base coat is adversely affected by theclear top coat, that is to say the orientation of the aluminum flakes isdisturbed, with the result that a coating film having an aluminum gloss(glitters, white reflections) cannot be obtained. When a thermosettingpolyester resin/melamine resin-based coating composition is used as themetallic base coat composition, the clear top coat begins to cure beforethe start of curing of said metallic base coat, and thisdisadvantageously causes such defects as ebullition and shrinkage of thecoat.

When a finish coat film has a defect, the defect is generally repairedby recoating with a metallic base coat and a clear top coat each of thesame kind as before. With the silicone-based coating compositionmentioned above, there remains a problem that the mutual adhesionbetween coating films (i.e. between top clear coat and metallic basecoat for recoating) is insufficient.

It is an object of the present invention to provide a method of formingcoating films excellent in finish and recoatability.

Other objects as well as features of the invention will become apparentfrom the description which follows.

The invention provides a method of forming coating films by forming insequence a pigmented base coat and a clear top coat on a substratefollowed by finishing by the two-coat one-bake technique, the methodbeing characterized by, as a coating composition for pigmented base coatformation, a composition comprising, as essential components thereof,

(1) an OH-containing resin,

(2) an amino resin,

(3) a polyorganosiloxane which has, on an average, at least two groups,per molecule, each selected from the class consisting of a silanol groupand an alkoxysilane group and has a number average molecular weight ofat least 1,000,

(4) a flaky metal powder and/or a mica powder and

(5) an organic solvent,

and using, as a coating composition for clear top coat formation, acomposition comprising, as essential components thereof,

(1) an OH-containing base resin which further contains at least onegroup selected from the class consisting of a silanol group and ahydrolyzable group bound directly to a silicon atom,

(2) an amino resin, and

(3) an organic solvent.

The invention is now described in more detail in the following.

Pigmented base coat

In the coating method of this invention, the pigmented base coat isformed from a base coat composition which comprises, as essentialcomponents thereof, (1) an OH-containing resin (hereinafter sometimesreferred to as "OH-containing base coat resin"), (2) an amino resin, (3)a polyorganosiloxane having, on an average, at least two groups eachselected from the group consisting of a silanol group and analkoxysilane group and having a number average molecular weight of atleast 1,000 (hereinafter sometimes referred to as "polyorganosiloxane")for short), (4) a flaky metal powder and/or a mica powder (hereinaftersometimes referred to as "metal flake" for short) and (5) an organicsolvent.

Preferred as the OH-containing base coat resin are OH-containingpolyester resins and OH-containing vinyl resins, among others.

The OH-containing polyester resins preferably have a hydroxyl value ofabout 20 to 200, more preferably about 50 to 150. When the hydroxylvalue is smaller than 20, the rate of curing of the base coat filmbecomes slower than that of the top coat film, so that the curing of thetop coat film goes ahead of that of the base coat film. As a result, thetop coat film tends to develop defects such as shrinkage, leading toimpairment of the appearance of the finished film. Furthermore, thecuring can hardly be complete, so that the performance characteristics(e.g. water resistance, impact resistance) of the coating film willunfavorably be deteriorated. On the other hand, hydroxyl valuesexceeding about 200 are undesirable since, in that case, a large numberof unreacted hydroxyl groups remain within the coating film, leading todeteriorated performance characteristics (e.g. water resistance, weatherresistance ) of the coating film.

Said OH-containing polyester resins may contain one or more carboxylgroups within the molecule. In particular, the carboxyl group canincrease the rate of reaction between the hydroxyl group and the aminogroup, between the hydroxyl group and the silanol or alkoxysilane group,or between one silanol or alkoxysilane group and another silanol oralkoxysilane group, to thereby effectively improve the finish of thecoating film. Recommendably, the carboxyl group content shouldcorrespond to about 0 to 50, preferably about 5 to 20 in an acid valueof resin.

As examples of the OH-containing polyester resins, there may bementioned oil-free alkyd resins derived from at least one polybasic acidand at least one polyhydric alcohol and, if necessary, an aromaticmonobasic acid; and alkyd resins derived from at least one polybasicacid and at least one polyhydric alcohol and, if necessary, an aromaticmonobasic acid plus an aliphatic monobasic acid.

The polybasic acid, polyhydric alcohol, aromatic monobasic acid andaliphatic monobasic acid that are to be used in the production of said(oil-free) alkyd resins are more specifically mentioned below.

As the polybasic acid, there may be mentioned, for example, phthalicacid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid,tetrahydroterephthalic acid, hexahydrophthalic acid,hexahydroisophthalic acid, hexahydroterephthalic acid, chlorendic acid,trimellitic acid, hexahydrotrimellitic acid, pyromellitic acid,cyclohexanetetracarboxylic acid, methyltetrahydrophthalic acid,methylhexahydrophthalic acid, endomethylenehexahydrophthalic acid,methylendomethylenetetrahydrophthalic acid, maleic acid, fumaric acid,itaconic acid, succinic acid, glutaric acid, adipic acid, azelaic acid,sebacic acid, decanedicarboxylic acid, suberic acid, pimelic acid, dimeracids (dimers of tall oil fatty acid), tetrachlorophthalic acid,naphthalenedicarboxylic acid, 4,4 '-diphenylmethanedicarboxylic acid and4,4 '-dicarboxybiphenyl, acid anhydrides of these, and dialkyl esters ofthese, in particular dimethyl esters.

As the polyhydric alcohol, there may be mentioned, among others,ethylene glycol, propylene glycol, 1,4-butanediol, 1,3-butanediol,2,3-butanediol, 1,2-butanediol, 1,5-pentanediol, 1,4-pentanediol,2,4-pentanediol, 2,3-dimethyltrimethylene glycol,3-methylpentane-1,5-diol, 3-methyl-4,5-pentanediol,2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol,1,4-hexanediol, 2,5-hexanediol, 1,4-cyclohexanedimethanol, diethyleneglycol, dipropylene glycol, triethylene glycol, neopentyl glycol,hydroxypivalic acid neopentyl glycol ester, polyalkylene oxides,bishydroxyethyl terephthalate, (hydrogenated) bisphenol A-alkylene oxideadducts, glycerol, trimethylolpropane, trimethylolethane, diglycerol,pentaerythritol, dipentaerythritol and sorbitol. Monoepoxy compoundssuch as Cardura E 10 (Shell Chemical Company), α-olefin epoxides andbutylene oxide may also be used as a kind of glycol.

Furthermore, compounds containing both carboxyl and hydroxyl groupswithin the molecule may also be used. As such compounds, there may bementioned, for example, dimethylolpropionic acid, pivalic acid,12-hydroxystearic acid and ricinolic acid. Lactones such asε-caprolactone and γ-valerolactone also fall under the category of thecompounds mentioned above since they are cyclic ester compounds.

The aromatic monobasic acid includes, among others, benzoic acid andp-tert-butylbenzoic acid.

The aliphatic monobasic acid includes, among others, caproic acid,captic acid, lauric acid, myristic acid, palmitic acid, stearic acid,isononanoic acid and coconut oil (fatty acids).

The (oil-free) alkyd resins mentioned above can be produced by any ofthe conventional methods, for example by subjecting a mixture of theabove-mentioned polybasic acid and polyhydric alcohol and, if necessary,monobasic acid to esterification or transesterification in the presenceof an esterification catalyst (e.g. dibutyltin dilaurate). Saidpolybasic acid and polyhydric alcohol are desirably used in proportionssuch that the number of moles of the polybasic acid component(s) iswithin the range of about 0.7 to 0.99, preferably about 0.8 to 0.98, permole of the polyhydric alcohol component(s). The esterification catalystis desirably used in an amount of about 0.1 to 1.0 part by weight,preferably about 0.2 to 0.5 part by weight, per 100 parts by weight ofthe sum total of the polyhydric alcohol and polybasic acid components.As for the reaction conditions, the reaction temperature is generallyabout 160° to 280° C., preferably about 180° to 260° C., and thereaction period is generally about 5 to 12 hours, preferably about 6 to8 hours.

As the OH-containing polyester resin, vinyl-modified alkyd resins, forinstance, may also be used. Usable as said vinyl-modified alkyd resinsare, for example, reaction products from an OH-containing (oil-free)alkyd resin and a COOH-- or NCO-containing vinyl resin, reactionproducts from an OH-- and COOH-containing (oil-free) alkyd resin and anepoxy-containing vinyl resin, products of radical polymerization of avinyl monomer [e.g. a polymerizable unsaturated monomer (b) mentionedbelow] in the presence of an (oil-free) alkyd resin having aradical-polymerizable unsaturated group [e.g. an alkyd resin containinga drying oil as an essential component, an alkyd resin derived from anOH-- and COOH-containing (oil-free) alkyd resin by reaction withglycidyl (meth)acrylate], and products of polymerization of a vinylmonomer [e.g. a polymerizable unsaturated monomer (b) mentioned below]in the presence of an (oil-free) alkyd resin or vinyl-modified alkydresin such as mentioned above, for instance, as a suspension stabilizer,in an organic solvent in which said monomer and suspension stabilizercan be dissolved but the polymer particles obtained from said monomerwill not be dissolved.

The molecular weight of the OH-containing polyester resin, as expressedin terms of number average molecular weight, can be within the range ofabout 500 to 10,000 in the case of an (oil-free) alkyd resin, or withinthe range of about 2,000 to 80,000 in the case of a vinyl-modified alkydresin. When said molecular weight is below the above range, the meltviscosity of the base coat decreases during baking, whereby theorientation of metal flakes changes, making it difficult to obtaincoating films with a metallic tone. A molecular weight exceeding theabove range is also undesirable since the workability in coatinggenerally decreases.

The OH-containing vinyl resins should preferably have a hydroxyl valueof about 20 to 200, more preferably about 50 to 150. When the hydroxylvalue is below 20, the rate of curing of the base coat film becomesslower than that of the top coat film, so that the curing of the topcoat film precedes the curing of the base coat film. As a result, thetop coat film tends to develop defects such as shrinkage, causing thefinished coating film to have an impaired appearance. Furthermore, thecuring can hardly be complete, so that the performance characteristics(e.g. water resistance, impact resistance) of the coating film willunfavorably be deteriorated. Hydroxyl values higher than about 200 arealso undesirable since, in that case, a large number of unreactedhydroxyl groups remain in the coating film, leading to deterioratedperformance characteristics (e.g. water resistance, weather resistance)of the coating film.

Said OH-containing vinyl resins may contain one or more carboxyl groupswithin the molecule. In particular, the carboxyl group can increase therate of reaction between the hydroxyl group and the amino group, betweenthe hydroxyl group and the silanol or alkoxysilane group, and betweenone silanol or alkoxysilane group and another silanol or alkoxysilanegroup, to thereby effectively improve the finish of the coating film.The carboxyl group content should recommendably correspond to about 0 to50, preferably about 5 to 20 in an acid value of resin.

Usable as the OH-containing vinyl resins are, for example, (co)polymersobtained by radical polymerization of at least one of the OH-containingvinyl monomers (a) mentioned below, optionally together with at leastone of other polymerizable unsaturated monomers (b) mentioned below.

As said OH-containing vinyl monomers (a), there may be mentioned, forexample, the following (a-1) to (a-5).

(a-1) Hydroxyalkyl vinyl ethers, such as hydroxybutyl vinyl ether;

(a-2) Allyl alcohol and methallyl alcohol;

(a-3) (Meth)acrylic acid hydroxyalkyl esters, such as hydroxyethyl(meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl(meth)acrylate;

(a-4) (Poly)alkylene glycol monoacrylates, such as ethylene glycolmonoacrylate, and polyethylene glycol monoacrylate;

(a-5) Adducts of (a-1) to (a-4) with a lactone (e.g. ε-caprolactone,γ-valerolactone).

As other polymerizable unsaturated monomers (b), there may be mentioned,for example, C₁₋₂₄ alkyl or cycloalkyl esters of (meth)acrylic acid,such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl(meth)acrylate, hexyl (meth)acrylate, octyl (meth)acrylate, lauryl(meth)acrylate and cyclohexyl (meth)acrylate; COOH-- containingcompounds such as (meth)acrylic acid, crotonic acid, itaconic acid,maleic acid (anhydride), fumaric acid and 2-carboxyethyl (meth)acrylate;aromatic vinyl compounds such as styrene and vinyltoluene;perfluoroalkyl (meth)acrylates such as perfluorobutylethyl(meth)acrylate, perfluoroisononylethyl (meth)-acrylate andperfluorooctylethyl (meth)acrylate; (meth)acrylonitrile; olefins;fluoroolefins; vinyl esters; cyclohexyl or alkyl vinyl ethers; and allylethers.

Said OH-containing vinyl resins can be produced in the conventionalmanner, for example by subjecting the above monomer or monomers (a),optionally together with one or more other monomers (b), topolymerization in a substantially inert organic solvent in the presenceof a radical polymerization initiator at about 80° to 220° C. for about4 to 18 hours.

As the organic solvent, there may be mentioned, for example, aromatichydrocarbons such as xylene and toluene; esters such as ethyl acetate,propyl acetate and butyl acetate; ketones such as acetone and methylethyl ketone; and ethers such as ethylene glycol, cellosolve,butylcellosolve and cellosolve acetate. Such organic solvents may beused either singly or as a mixture of two or more of them.

Radical polymerization initiators which can be used include, amongothers, azo type initiators such as 2,2'-azobisisobutyronitrile and2,2'-azobis ( 2,4-dimethylvaleronitrile), and peroxide type initiatorssuch as benzoyl peroxide, lauryl peroxide and tert-butyl peroctoate. TheOH-containing vinyl resins may have a number average molecular weight ofabout 2,000 to 80,000, preferably about 4,000 to 20,000. When themolecular weight is smaller than about 2,000, the melt viscosity of thebase coat decreases excessively during baking, allowing the orientationof metal flakes to change. As a result, coating films with a metallictone can hardly be obtained. Molecular weights higher than about 80,000are also undesirable since the workability in coating decreases.

The amino resin is used as a crosslinking agent and includes methylolamino resins obtained by reacting an aldehyde with an amino componentsuch as melamine, urea, benzoguanamine, acetoguanamine, stearoguanamine,spiroguanamine or dicyandiamide. The aldehyde is, for example,formaldehyde, paraformaldehyde, acetaldehyde, or benzaldehyde. Productsof etherification of these methylol amino resins with an alcohol mayalso be used. Examples of the alcohol to be used for etherification aremethyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol,n-butyl alcohol, isobutyl alcohol, 2-ethylbutanol and 2-ethylhexanol,among others. Highly etherified melamine resins, namely melamine resinshaving, on an average, at least three methyl ether moieties per triazinering or melamine resins derived therefrom by partial transetherificationof the methoxy groups with an alcohol containing 2 or more carbon atoms,can be used as said amino resins. Among these, low-molecular-weightmelamines having an average condensation degree of not more than 2 withsingle nucleus forms accounting for at least 50% by weight are preferredsince high solids contents of the base coat composition can be attainedwith them. When such melamines are used, a conventional curing catalystsuch as p-toluenesulfonic acid should preferably be added.

The polyorganosiloxane is used as a crosslinking agent for theOH-containing base coat resin, like the amino resin mentioned above, andfurther as a self-curing resin, and contains, on an average, at leasttwo groups, preferably 2 to 10 groups, per molecule, each selected fromamong silanol and alkoxysilane groups. It suitably has a number averagemolecular weight of not less than 1,000, preferably 1,000 to 50,000. Theincorporation of a polyorganosiloxane leads to improved curability ofthe interface layer (mixed layer) between the pigmented base coat andthe clear top coat, whereby such coating film performancecharacteristics as finish appearance and solvent resistance can beimproved. The polyorganosiloxane preferably contains a ladder structureportion and can be prepared, for example by subjecting a silane compoundessentially comprising a trifunctional silane of the general formula

    R.sup.1.sub.x Si(OR.sup.2).sub.4-x                         (I)

wherein R¹ and R² may be the same or different and each is a hydrocarbongroup containing 1 to 13 carbon atoms and x is 1, to hydrolysiscondensation.

As the R¹ and R² groups mentioned above, there may be mentioned, forexample, methyl, ethyl, propyl, butyl and phenyl.

The silane compound to be subjected to hydrolysis condensation may becomposed of a trifunctional silane of formula (I) alone or may contain,in addition to the trifunctional silane, a bifunctional ormonofunctional silane in which x in formula (I) has a value of 2 or 3,respectively, or both. In said silane compound, the content of thetrifunctional silane of formula (I) should preferably be not less than30% by weight, more preferably not less than 80% by weight.

As typical examples of the trifunctional silane of formula (I), theremay be mentioned methyltrimethoxysilane, methyltriethoxysilane,phenyltrimethoxysilane, phenyltriethoxysilane, isobutyltrimethoxysilane,isobutyltriethoxysilane and ethyltrimethoxysilane. As the bifunctionalor monofunctional silane which can be used in admixture with thetrifunctional silane of formula (I), there may be mentioned, forexample, dimethyldimethoxysilane, diphenyldimethoxysilane,diphenyldiethoxysilane, diisobutyldimethoxysilane,diisobutyldipropoxysilane and trimethylmethoxysilane.

In the practice of the invention, the polyorganosiloxane shouldpreferably contain, on an average, not only at least two groups permolecule each selected from among silanol and alkoxysilane groups butalso at least one epoxy group, preferably 1 to 30 epoxy groups, permolecule. By using an epoxy-containing polyorganosiloxane, thecurability of the mixed layer can be further improved to give coatingfilms very excellent in finish appearance, solvent resistance and soforth.

The epoxy-containing polyorganosiloxane can be prepared by subjecting tohydrolysis cocondensation a silane representable by the general formula(I) wherein x is 1, 2 or 3 and an epoxy-containing silane of the generalformula ##STR1## wherein R³ and R⁴ may be the same or different and eachis a hydrocarbon group containing 1 to 13 carbon atoms, y is 1, 2 or 3,G is a group of the formula (III ) or (IV) given below. ##STR2## In theabove formulas (III)and (IV), R⁵ is a bivalent hydrocarbon groupcontaining 1 to 13 carbon atoms and the R⁶ groups may be the same ordifferent and each is a hydrogen atom or a methyl group.

As the R³ and R⁴ groups mentioned above, there may be mentioned, forexample, methyl, ethyl, propyl, butyl and phenyl. As the R⁵ groupmentioned above, there may be mentioned, for example, methylene,ethylene, propylene, butylene and hexamethylene.

The proportions of the silane of formula (I) in which x is 1, 2 or 3 andthe silane of formula (II) to be subjected to cocondensation are notcritical but, generally, the silane (I):silane (II) weight ratio iswithin the range of 95:5 to 10:90. It is preferable that, among thesilane species to be charged, the silane of formula (I) in which x is 1and the silane of formula (II) in which y is 1 combinedly account fornot less than 30% by weight, more preferably not less than 80% byweight.

Typical examples of the silane of formula (I) which is to be used forcocondensation with the epoxy-containing silane of general formula (II)include, in addition to the above-mentioned silanes in which x is 1,silanes in which x is 2 or 3, such as dimethyldimethoxysilane,diphenyldimethoxysilane, diphenyldiethoxysilane,diisobutyldimethoxysilane, diisobutyldipropoxysilane andtrimethylmethoxysilane.

Typical examples of the silane of formula (II) areγ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane,β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane andβ-(3,4-epoxycyclohexyl)ethyltriethoxysilane.

The hydrolysis condensation or cocondensation of the silane compoundsmentioned above is carried out by mixing the silane compound(s) with awater-soluble solvent (e.g. alcohol type or cellosolve type solvent) asnecessary and allowing the hydrolysis and condensation reactions toproceed in the presence of an inorganic acid such as hydrochloric acid,sulfuric acid or phosphoric acid or an organic acid such as formic acidor acetic acid and in the presence of water, preferably at a pH of notmore than 6, at a temperature of about 20° to 100° C. with stirring forabout 30 minutes to 20 hours.

The molecular weight of the polyorganosiloxane can suitably becontrolled by adjusting or selecting the amount o f water, the kind andamount of catalyst, the reaction temperature and the reaction time,among others.

When, in the above-mentioned polyorganosiloxane, the average number ofsilanol and/or alkoxysilane groups per molecule is below 2, the rate ofcuring of the base coat film becomes slower than that of the top coatfilm and the curing of the top coat film begins before curing of thebase coat film, so that the top coat film develops defects such asshrinkage and the finished coating film presents an unsatisfactoryappearance.

When the polyorganosiloxane resin has a number average molecular weightof less than 1,000, the melt viscosity of the base coat becomesexcessively low during baking and, as a result, the orientation of metalflakes is disturbed, so that coating films with a metallic tone canhardly be obtained.

In the base coat composition, the proportions of the OH-containing basecoat resin, amino resin and polyorganosiloxane, which are to serve asbinders, based on the total amount (resin solids content) of said threecomponents, are recommendably as follows: OH-containing base coat resin5 to 90% by weight, preferably 30 to 60% by weight, amino resin 5 to 50%by weight, preferably 10 to 40% by weight, and polyorganosiloxane 1 to40% by weight, preferably 5 to 30% by weight, more preferably 5 to 20%by weight.

When the OH-containing base coat resin is used in an amount of less than5% by weight, the adhesion to substrates becomes insufficient, while,when said amount is above 90% by weight, the appearance and performancecharacteristics (e.g. water resistance, processability) of the finishedcoating film become unsatisfactory. When the proportion of the aminoresin is below 5% by weight, the crosslink density of the OH-containingresin becomes low and the water resistance and impact resistance becomemarkedly poor. When said proportion exceeds 50% by weight, themechanical characteristics and the adhesion to substrates arecompromised. When the proportion of the polyorganosiloxane is below 1%by weight, the accelerating effect on the rate of curing of the basecoat film is slight, hence the finished coating film cannot be improvedin appearance, whereas, when said proportion exceeds 40% by weight, theadhesion to substrates becomes unsatisfactory.

The metal flakes are used to give a metallic tone to coating films. Asthe flaky metal, there may be mentioned, for example, aluminum flakes,nickel flakes, copper flakes, brass flakes and chromium flakes. As themica powder, there may be mentioned pearl mica and pigmented pearl mica,among others.

In the base coat composition, metal flakes are incorporated in an amountof about 1 to 20 parts by weight per 100 parts by weight of resinsolids.

As examples of the organic solvent, there may be mentioned aromatichydrocarbons such as xylene and toluene, esters such as ethyl acetate,propyl acetate and butyl acetate, ketones such as acetone and methylethyl ketone, and ethers such as ethylene glycol, cellosolve,butylcellosolve and cellosolve acetate. Such organic solvents may beused either singly or in the form of a mixture of two or more of them.From the standpoint of finishing performance, the boiling point shouldpreferably be not higher than about 150° C. but this is not critical.

Other additives for coating composition than those mentioned above whichcan be incorporated in the base coat composition as necessary includeorganic pigments, inorganic pigments, pigment dispersing agents, finelydivided polymers, ultraviolet absorbers, coated surface modifiers,curing catalysts, cellulose acetate (and derivatives thereof), etc.

As pigments which may be incorporated in said coating composition, theremay be mentioned, for example, organic pigments (e.g. quinacridonepigments such as quinacridone red, azo pigments such as pigment red,phthalocyanine pigments such as phthalocyanine blue and phthalocyaninegreen), inorganic pigments (e.g. titanium oxide, barium sulfate, calciumcarbonate, baryta, clay, silica), and carbon-based pigments (e.g. carbonblack).

The base coat composition to be used in the practice of the inventiongenerally has a resin solids content of about 10 to 50% by weight.

Clear top coat

The top coat is formed from a clear coating composition (top coatcomposition) comprising, as essential components thereof, anOH-containing base resin (hereinafter referred to as "top coat baseresin") which further contains one or more groups each selected from thegroup consisting of a silanol group and a hydrolyzable group bounddirectly to a silicon atom (hereinafter referred to as "hydrolyzablesilyl group"), an amino resin and an organic solvent.

The hydrolyzable group in the above top coat base resin is a groupcapable of undergoing hydrolysis in the presence of water to form asilanol group. Said group includes, among others, C₁₋₅ alkoxy groups;aryloxy groups such as phenoxy, tolyloxy, p-methoxyphenoxy,p-nitrophenoxy and benzyloxy; acyloxy groups such as acetoxy,propionyloxy, butanoyloxy, benzoyloxy, phenylacetoxy and formyloxy; andresidues of the formula --N(R⁷)₂, --ON(R⁷)₂, --ON═C(R⁷)₂ or --NR⁸ COR⁷(in which the R⁷ groups are the same or different and each is C₁₋₈alkyl, aryl or aralkyl and R⁸ is H or C₁₋₈ alkyl).

The top coat base resin to be used in the practice of the invention maybe an OH-containing resin which further contains at least one groupselected from the class consisting of a silanol group and a hydrolyzablesilyl group as an additional essential functional group component withinthe same molecule (hereinafter said resin being sometimes referred to as"single resin") or a mixed resin composed of a resin having at least onegroup selected from the class consisting of a silanol group and ahydrolyzable silyl group [hereinafter said resin being sometimesreferred to as "resin (or polymer) A"] and an OH-containing resin[hereinafter sometimes referred to as "resin (or polymer) B"]. In kind,said resins include vinyl polymers, polyester resins and polyetherpolyester resins, among others. These resins should desirably have anumber average molecular weight of about 2,000 to 100,000 preferablyabout 4,000 to 80,000, in the case of vinyl polymers, about 500 to20,000, preferably about 1,000 to 5,000, in the case of polyester andpolyether polyester resins. When the number average molecular weight isbelow the lower limit mentioned above, the paint film surface tends todevelop such defects as shrinkage. When the number average molecularweight is above the upper limit, the workability in painting will beinferior and the paint film surface tends to develop such defects asorange peel.

Among the resins mentioned above, vinyl polymers, which are excellent inperformance characteristics such as weather resistance, acid resistanceand mar resistance, are preferred as the top coat base resin to be usedin the practice of the invention. Such vinyl polymers are mentionedbelow in further detail.

As examples of the single resin of the vinyl polymer type, there may bementioned vinyl copolymers produced by copoymerizing a silanol- and/orhydrolyzable silyl-containing vinyl monomer (c) and an OH-containingvinyl monomer (a), optionally together with some other polymerizableunsaturated monomer (b), and polymers produced by first preparing afunctional group-containing vinyl polymer and then reacting said vinylpolymer with a silanol- and/or hydrolyzable silyl-containing silanecompound which further has a group complementarily reacting with thefunctional group of said vinyl polymer (the silanol and/or hydrolyzablesilyl group may be the complementarily reacting group), for examplepolymers produced by reacting a copolymer of glycidyl (meth)acrylate andan OH-containing vinyl monomer (a) and some other vinyl monomer (b) withγ-aminopropyltrimethoxysilane and polymers produced by reacting acopolymer of (meth)acrylic acid, an OH-containing vinyl monomer (a) andsome other polymerizable unsaturated monomer (b) withγ-glycidoxypropyltrimethoxysilane.

Said vinyl polymers should desirably have, on an average, at least onesilanol and/or hydrolyzable silyl group per molecule. When the averagenumber of silanol and/or hydrolyzable silyl groups is less than one permolecule, the curability will be unsatisfactorily low and coating filmsexcellent in weather resistance, acid resistance, mar resistance andother characteristics can hardly be formed. Furthermore, said vinylpolymers should desirably have hydroxyl groups in a content such thatthe hydroxyl value amounts to about 30 to 300, preferably about 50 to200. When the hydroxyl value is below about 30, the curability will below and coating films excellent in weather resistance, acid resistance,mar resistance and other characteristics will not be obtained.Conversely, when the hydroxyl value exceeds 300, a large number ofunreacted hydroxyl groups will remain in the coating film, unfavorablyleading to decreased water resistance, for instance.

As examples of the mixed resin of the vinyl polymer type, there may bementioned, as polymer A, vinyl polymers produced by (co)polymerizing asilanol- and/or hydrolyzable silyl-containing vinyl monomer (c),optionally with another polymerizable unsaturated monomer (b) andpolymers produced by first preparing a functional group-containing vinylpolymer and then reacting said vinyl polymer with a silanol- and/orhydrolyzable silylcontaining silane compound which further has a groupcomplementarily reacting with the functional group of said vinyl polymer(the silanol and/or hydrolyzable silyl group may be the complementarilyreacting group), for example polymers produced by reacting a copolymerof glycidyl (meth)acrylate and some other polymerizable unsaturatedmonomer (b) with γ-aminopropyltrimethoxysilane and polymers produced byreacting a copolymer of (meth)acrylic acid and some other polymerizableunsaturated monomer (b) with γ-glycidoxypropyltrimethoxysilane, and, aspolymer B, polymers produced by polymerizing an OH-containing vinylmonomer (a) and copolymers of said vinyl monomer (a) and some otherpolymerizable unsaturated monomer (b).

Said polymer A should desirably have, on an average, at least onesilanol and/or hydrolyzable silyl group per molecule. When the averagenumber of silanol and/or hydrolyzable silyl groups is less than one permolecule, the curability will be low and it will be impossible to formcoating films excellent in weather resistance, acid resistance, marresistance, etc.

Said polymer B should preferably have a hydroxyl value of not less thanabout 30. When the hydroxyl value is less than about 30, the curabilitywill be low and paint films excellent in weather resistance, acidresistance, mar resistance and other characteristics can hardly beformed. When the hydroxyl value is excessively high, a large number ofunreacted hydroxyl groups will remain in the coating film, causing thewater resistance and other properties of the coating film to decrease.Hence, the hydroxyl value should preferably be not more than about 300.

Said polymer A and polymer B are admixed in a polymer A/polymer B weightratio of 90:10 to 10:90, preferably 80:20 to 20:80.

Among the single and mixed resins mentioned above, single resins, whichcan readily form coating films excellent in appearance and performancecharacteristics with uniform curing, are preferred.

As the monomers (a) and (b), there may be mentioned those alreadymentioned hereinbefore.

As the vinyl monomer (c) mentioned above, there may be mentioned, forexample, acrylic silane compounds such as γ-(meth)acryloxyethyltrimethoxysilane, γ-(meth)-acryloxypropyltrimethoxysilane,γ-(meth )acryloxypropyltriethoxysilane,γ-(meth)acryloxypropyltripropoxysilane, γ-(meth)acryloxypropylmethyldimethoxysilane,γ-(meth)-acryloxypropylmethyldiethoxysilane,γ-(meth)acryloxypropylmethyldipropoxysilane,δ-(meth)acryloxybutylphenyldimethoxysilane,δ-(meth)acryloxybutylphenyldiethoxysilane,δ-(meth)acryloxybutylphenyldipropoxysilane,γ-(meth)acryloxypropyldimethylmethoxysilane,γ-(meth)acryloxypropyldimethylethoxysilane,γ-(meth)acryloxypropylphenylmethylmethoxysilane,γ-(meth)acryloxypropylphenylmethylethoxysilane andγ-(meth)acryloxypropyltriacetoxysilane; vinylsilane compounds such asvinyltrimethoxysilane and vinyltriethoxysilane; allylsilane compoundssuch as allyltriethoxysilane; and styrylsilane compounds such as2-styrylethyltrimethoxysilane.

The top coat base resin to be used in the practice of the invention maybe a COOH-containing one. A method of introducing the carboxyl groupinto said resin comprises, for instance, using a COOH-containing vinylmonomer as the other vinyl monomer (b) component in the vinyl polymerproduction. Such monomer, when employed, is used generally in an amountsuch that the COOH-containing vinyl monomer amounts to about 5 to 50% byweight, preferably about 10 to 40% by weight, on the vinyl polymerbasis.

The amino resin to be used in combination with the above-mentioned topcoat base resin includes, among others, those methylol amino resins andetherified amino resins (derived from methylol amino resins byetherification with an alcohol) mentioned above for the pigmented basecoat paint composition.

Said amino resin is used suitably in an amount of about 2 to 100 partsby weight per 100 parts by weight of the base resin. When its amount isless than 2 parts by weight, the water resistance and othercharacteristics will be unsatisfactory, while, when its amount is above100 parts by weight, the weather resistance will be adversely affected.

Useful as the organic solvent are, for example, hydrocarbon solventssuch as heptane, toluene, xylene, octane and mineral spirit; estersolvents such as ethyl acetate, n-butyl acetate, isobutyl acetate,methylcellosolve acetate and butylcarbitol acetate; ketone solvents suchas methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone;alcohol solvents such as ethanol, isopropanol, n-butanol, sec-butanoland isobutanol; and ether solvents such as n-butyl ether, dioxane,ethylene glycol monomethyl ether and ethylene glycol monoethyl ether.

The clear coating composition to be used in clear top coat formation inaccordance with the invention can be prepared by dissolving ordispersing the above-mentioned top coat base resin and amino resin in anorganic solvent. Said composition may be in the form of a nonaqueousdispersion coating composition.

Said clear coating composition may contain, when necessary, a curingcatalyst for the silanol and/or hydrolyzable silyl group, in addition tothe above-mentioned base resins and organic solvent.

As the curing catalyst, there may be mentioned, for example, acidiccompounds such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid,trichloroacetic acid, phosphoric acid, mono-n-propyl phosphate,monoisopropyl phosphate, mono-n-butyl phosphate, monoisobutyl phosphate,mono-tert-butyl phosphate, monooctyl phosphate, monodecyl phosphate,other monoalkyl phosphates, di-n-propyl phosphate, diisopropylphosphate, di-n-butyl phosphate, diisobutyl phosphate, di-tert-butylphosphate, dioctyl phosphate, didecyl phosphate, other dialkylphosphates, β-hydroxyethyl (meth)acrylate phosphate, mono-n-propylphosphite, monoisopropyl phosphite, mono-n-butyl phosphite, monoisobutylphosphite, mono-tert-butyl phosphite, monooctyl phosphite, monodecylphosphite, other monoalkyl phosphites, di-n-propyl phosphite,diisopropyl phosphite, di-n-butyl phosphite, diisobutyl phosphite,di-tert-butyl phosphite, dioctyl phosphite, didecyl phosphite, otherdialkyl phosphites; tetraisopropyl titanate, tetrabutyl titanate, othertitanium-containing compounds; tin octanoate, dibutyltin diacetate,dibutyltin dioctanoate, dibutyltin dilaurate, dibutyltin dimaleate,other tincontaining compounds; and basic compounds such as butylamine,tert-butylamine, dibutylamine, hexylamine, ethylenediamine,triethylamine, isophoronediamine, imidazole, lithium hydroxide, sodiumhydroxide, potassium hydroxide and sodium methylate. At least one ofthese is used.

These curing catalysts may be used in an amount of about 0.05 to 20parts by weight, preferably about 0.1 to 10 parts by weight, per 100parts by weight of the top coat base resin.

When hexamethoxymethylmelamine or an etherified melamine resin derivedtherefrom by partial or complete substitution of the methoxy groups witha C₃ or higher alcohol is used as the amino resin, a strongly acidiccatalyst, such as p-toluenesulfonic acid or dodecylbenzenesulfonic acid,is generally used as the curing catalyst for the reaction of such resinwith the hydroxyl group. Said strongly acidic catalyst is neutralized(blocked) with an amine compound such as triethylamine, diethanolamineor 2-amino-2-methylpropanol for providing storage stability and enablingthe use of the coating composition as a one-pack type paint. Thisneutralized (blocked) strongly acidic catalyst can serve as a curingcatalyst for alkoxysilane groups as well. Thus, at a baking temperatureof 100° C. or above, the neutralized (blocked) strongly acidic catalystserves as a common catalyst for the reaction of resins and the reactionof alkoxysilanes.

The clear coating composition may contain, as necessary, ultravioletabsorbers, antioxidants, light stabilizers, fine polymer particles andvarious other additives for coating composition.

The clear coating composition to be used in the practice of theinvention should generally have a resin solids content of about 35 to70% by weight.

The method of this invention can be practiced by applying the pigmentedbase coat composition to a substrate such as a coated surface preparedby electrodeposition coating of a steel sheet (after chemical conversiontreatment) followed by or without a subsequent intercoating, or a coatedsurface prepared by applying a primer to any of various plasticmaterials with or without subsequent intercoating, and then applying thetop clear coat composition. The electrodeposition coating compositionand the intercoating composition are baked generally at 140° to 190° C.for 30 to 90 minutes, although the baking conditions may vary dependingon the composition types. The pigmented base coat composition and topclear coat composition can be applied in the conventional manner, forexample using an electrostatic or non-electrostatic coating machine orequipment. The thickness of the pigmented base coat film is preferablyabout 10 to 50 μm (after curing). After application of base coatcomposition, the coating film is dried by allowing to stand at roomtemperature for several minutes or by forced drying at about 50° to 80°C. for several minutes, and then the clear coat composition is applied.The clear coat film preferably has a thickness of 20 to 100 μm (aftercuring). The coated material is heated at about 120° to 180° C. forabout 30 to 90 minutes, whereby curing can be attained.

In the method of this invention, the pigmented base coat composition tobe finished by the two-coat one-bake technique contains, as a bindercomponent, a polyorganosiloxane having a number average molecular weightof at least 1,000 and having at least two groups, per molecule, whichare each selected from the class consisting of silanol group and analkoxysilane group, so that, upon heating, the reaction between silanolgroups (inclusive of silanol groups resulting from hydrolysis ofalkoxysilane groups) and the reaction of a silanol group, an amino groupin the amino resin and a hydroxyl group in the OH-containing resin occurrapidly. Said reactions proceed faster than the curing reaction of theclear coating composition, so that, in the early stage of baking, theviscosity of the base coat composition increases more rapidly than thatof the clear coat composition. It is presumable that, for this reason, abase coat film durable to the shrinking force of the clear coat film asresulting from curing of the clear coating composition and volume changethereof due to solvent evaporation is formed and, as a result, thecoating film acquires an improved finish appearance. Furthermore, thecoating film formed shows excellent coating film performancecharacteristics (solvent resistance, weather resistance, mar resistanceetc.). In addition, even when said pigmented base coat composition isapplied again to the cured clear coat film for recoating, an improvedadhesion between both the coating films can be observed.

The following examples are further illustrative of the presentinvention.

In the examples, "part(s)" and "%" are "part(s) by weight" and "% byweight", respectively.

PRODUCTION EXAMPLE 1

A reaction vessel was charged with 0.29 mole of isophthalic acid, 0.23moleof phthalic acid, 0.43 mole of hexahydrophthalic acid, 0.4 mole oftrimethylolpropane, 0.6 mole of neopentyl glycol and 0.1 mole of coconutoil fatty acid and the condensation polymerization was carried out at200°-230° C. to give a coconut oil-modified polyester resin with an acidvalue of 8 and a hydroxyl value of 72. A 60% resin solution (a-1) wasprepared by adding 43 parts of xylene to 100 parts of said polyesterresin. Its viscosity was Y (25° C., Gardner bubble viscosity).

PRODUCTION EXAMPLE 2

An OH-containing acrylic resin solution (a-2) was prepared by reacting30 parts of methyl methacrylate, 53 parts of n-butyl methacrylate, 15parts of 2-hydroxyethyl acrylate and 2 parts of acrylic acid in anorganic solvent composed of 85 parts of xylene and 15 parts ofn-butanol. The number average molecular weight of the resin was 6,600,the resin solids content 50%, and the Gardner bubble viscosity V.

PRODUCTION EXAMPLE 3

An OH-containing acrylic resin solution (a-3) was prepared by reacting30 parts of methyl methacrylate, 45 parts of n-butyl methacrylate and 25parts of 2-hydroxyethyl acrylate in an organic solvent composed of 85parts of xylene and 15 parts of n-butanol. The number average molecularweight was 5,000, the resin solids content 50%, and the Gardner bubbleviscosity T.

    ______________________________________                                        Production Example 4                                                          ______________________________________                                        Phenyltrimethoxysilane 198    parts                                           Deionized water        54     parts                                           98% Sulfuric acid      0.002  part                                            ______________________________________                                    

The above materials were mixed up and subjected to reaction at 60° C.for 5 hours, the byproduct methanol was then distilled off under reducedpressure, and xylene was added to the remaining mixture to give apolyorganosiloxane solution (b-1) with a solids content of 50% and aGardner viscosity of AB. The polyorganosiloxane obtained had a numberaverage molecular weight of about 5,000 and had, on an average, 6silanol groups per molecule.

    ______________________________________                                        Production Example 5                                                          ______________________________________                                        Methyltrimethoxysilane  136 parts                                             Diphenyldimethoxysilane 182 parts                                             Deionized water          90 parts                                             60% Phosphoric acid      1 part                                               ______________________________________                                    

The above materials were mixed up and subjected to reaction at 60° C.for 10 hours, the byproduct methanol was distilled off under reducedpressure, and butyl acetate was added to the remaining mixture to give apolyorganosiloxane solution (b-2) with a solids content of 50% and aGardner viscosity of D. The polyorganosiloxane obtained had a numberaverage molecular weight of about 15,000 and had, on an average, 10silanol groups per molecule.

    ______________________________________                                        Production Example 6                                                          ______________________________________                                        Diphenyldimethoxysilane 182 parts                                             β-(3,4-Epoxycyclohexyl)-                                                                         186 parts                                             ethyltrimethoxysilane                                                         Deionized water         108 parts                                             60% Phosphoric acid      1 part                                               ______________________________________                                    

The above materials were mixed up and subjected to reaction at 60° C.for 15 hours, the byproduct methanol was then distilled off underreduced pressure, and butyl acetate was added to the remaining mixtureto give a polyorganosiloxane solution (b-3) with a solids content of 50%and a Gardner viscosity of G. The polyorganosiloxane obtained had anumber average molecular weight of about 2,000 and had, on an average,10 silanolgroups and 7 epoxy groups per molecule.

    ______________________________________                                        Production Example 7                                                          ______________________________________                                        Phenyltrimethoxysilane   198    parts                                         γ-Glycidoxypropyltrimethoxysilane                                                                236    parts                                         Deionized water          108    parts                                         60% Sulfuric acid        0.1    part                                          ______________________________________                                    

The above materials were mixed up and subjected to reaction at 60° C.for 10 hours, the byproduct methanol was distilled off under reducedpressure, and xylene was added to the remaining mixture to give apolyorganosiloxane solution (b-4) with a solids content of 50% and aGardner viscosity of DE. The polyorganosiloxane obtained had a numberaverage molecular weight of about 8,000 and had, on an average, 6silanol groups and 20 epoxy groups per molecule.

PIGMENTED BASE COAT COMPOSITION Base coat composition 1

A mixture of 100 parts of the resin solution (a-1) obtained inProduction Example 1 (60 parts as solids), 33.3 parts of U-Van 20SE(Note 1) (20 parts as solids), 40 parts of the polyorganosiloxanesolution (b-1) obtained in Production Example 4 (20 parts as solids), 20parts of an aluminum paste and 1 part of Reibo #3 (Note 2) was stirredand then a mixed solvent composed of Swazol #1000 (Note 3 ) and ethylacetate (20/80 by weight ) was added to adjust the viscosity of thecoating composition to 15 seconds (Ford cup #4/20° C.). The coatingcomposition thus prepared was subjected to testing.

(Note 1) U-Van 20SE, trademark of Mitsui Toatsu Chemicals, a butyletherized melamine resin solution with a solids content of about 60%.

(Note 2) Reibo #3, trademark of Reibo Chemical, a silicone-based surfacemodifier solution (active ingredient content about 1%).

(Note 3) Swazol #1000, trademark of Cosmo Oil Co., a petroleum-derivedaromatic solvent.

Base coat composition 2

Base coat composition 2 was prepared according to the same formulationas mentioned above for base coat composition 1 except that thepolyorganosiloxane solution (b-2) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 3

Base coat composition 3 was prepared according to the same formulationas mentioned above for base coat composition 1 except that thepolyorganosiloxane solution (b-3) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 4

Base coat composition 4 was prepared according to the same formulationas mentioned above for base coat composition 1 except that thepolyorganosiloxane solution (b-4) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 5

A mixture of 112 parts of the resin solution (a-2) obtained inProduction Example 2 (56 parts as solids), 40 parts of U-Van 20SE (24parts as solids), 40 parts of the polyorganosiloxane solution (b-1)obtained in Production Example 4 (20 parts as solids), 20 parts of analuminum paste and 1 part of Reibo #3 was stirred and then a 30:70 (byweight) mixture ofSwazol #1000 and ethyl acetate was added to adjust theviscosity of the coating composition to 15 seconds (Ford cup #4/20° C.).The resultant composition was subjected to testing.

Base coat composition 6

Base coat composition 6 was prepared according to the same formulationas mentioned above for base coat composition 5 except that the resinsolution(a-3) was used in lieu of the resin solution (a-2).

Base coat composition 7

Base coat composition 7 was prepared according to the same formulationas mentioned above for base coat composition 5 except that thepolyorganosiloxane solution (b-2) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 8

Base coat composition 8 was prepared according to the same formulationas mentioned above for base coat composition 5 except that thepolyorganosiloxane solution (b-3) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 9

Base coat composition 9 was prepared according to the same formulationas mentioned above for base coat composition 5 except that the resinsolution(a-3) was used in lieu of the resin solution (a-2) and that thepolyorganosiloxane solution (b-3) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 10

Base coat composition 10 was prepared according to the same formulationas mentioned above for base coat composition 5 except that thepolyorganosiloxane solution (b-4) was used in lieu of thepolyorganosiloxane solution (b-1).

Base coat composition 11

Base coat composition 11 was prepared according to the same formulationas mentioned above for base coat composition 1 except that thepolyorganosiloxane solution (b- 1) was not used.

Base coat composition 12

Base coat composition 12 was prepared according to the same formulationas mentioned above for base coat composition 5 except that thepolyorganosiloxane solution (b-1) was not used.

    ______________________________________                                        Production Example 8                                                          ______________________________________                                        Styrene                   100 parts                                           n-Butyl acrylate          450 parts                                           2-Ethylhexyl methacrylate 200 parts                                           2-Hydroxyethyl methacrylate                                                                             150 parts                                           γ-Methacryloxypropyltrimethoxysilane                                                              100 parts                                           Azobisisobutyronitirle     40 parts                                           ______________________________________                                    

A mixture composed of the above-materials was added dropwise to the samequantity (as said mixture) of xylene at 110° C. over 3 hours andmaturation was effected at the same temperature for 2 hours to give apolymer solution. The clear polymer obtained had a number averagemolecular weight of 6,000.

    ______________________________________                                        Production Example 9                                                          ______________________________________                                        Styrene                   150 parts                                           n-Butyl methacrylate      500 parts                                           1,4-Butanediol monoacrylate                                                                             200 parts                                           γ-Methacryloxypropyltrimethoxysilane                                                              150 parts                                           Azobisisobutyronitrile     40 parts                                           ______________________________________                                    

A mixture composed of the above materials was added dropwise to a mixedsolvent composed of 500 parts of xylene and 500 parts of n-butanol at120° C. over 3 hours, followed by 2 hours of maturation at thesametemperature, to give a polymer solution. The clear polymer obtainedhad a number average molecular weight of 6,000.

CLEAR TOP COAT COMPOSITION

    ______________________________________                                        Clear top coat composition 1                                                  ______________________________________                                        Solution of Production Example 8                                                                       140    parts                                         (solids content 50%)                                                          Cymel 303 (note 4)       30     parts                                         Nacure 5225 (note 5)     1.5    parts                                         Surface modifier (BYK Chemie's                                                                         0.1    part                                          BYK-300 solution; hereinafter                                                 the same shall apply)                                                         Ultraviolet absorber     1.0    part                                          (Ciba-Geigy's Tinuvin 900;                                                    hereinafter the same shall apply)                                             ______________________________________                                    

A mixture composed of the above materials was diluted with Swazol #1000to thereby adjust the viscosity (Ford cup #4, 20° C.) to 25 seconds.

(Note 4) Cymel 303: fully methoxylated melamine resin, available fromMitsui Cyanamid Ltd.

(Note 5) Nacure 5225: dimethyloxazolidineneutralizeddodecylbenzenesulfonicacid, available from King Industries.

    ______________________________________                                        Clear top coat composition 2                                                  ______________________________________                                        Solution of Production Example 9                                                                       160    parts                                         (solids content 50%)                                                          Cymel 303                20     parts                                         Nacure 5225              2      parts                                         Surface modifier         0.1    part                                          Ultraviolet absorber     1.0    part                                          ______________________________________                                    

The procedure for clear top coat composition 1 was followed using theabovematerials.

    ______________________________________                                        Clear top coat composition 3                                                  ______________________________________                                        Solution of Production Example 9                                                                       140    parts                                         (solids content 50%)                                                          60% U-Van 20SE           50     parts                                         Dibutyltin dilaurate     0.5    part                                          Surface modifier         0.1    part                                          Ultraviolet absorber     1.0    part                                          ______________________________________                                    

The procedure for clear top coat composition 1 was followed using theabovematerials.

EXAMPLES 1 TO 30 AND COMPARATIVE EXAMPLES 1 TO 6 Preparation of coatedsheets

An epoxy resin-based cationic electrodeposition coating film (25 μm)wasformed on dull steel sheets (after chemical conversion treatment)and, after curing thereof by heating at 170° C. for 30 minutes, LugaBake AM (trademark of Kansai Paint, an automotive coating composition ofthe polyester rein/melamine resin type) was applied to a film thicknessof30 μm (after drying) for intercoating, which was followed by 30minutes of baking at 140° C. The coated surface of each sheet wassubjectedto wet sanding using a #400 sand paper and, after draining anddrying, wiped using petroleum benzine, and used as a substrate.

Using the air spray coating technique, the pigmented base coatcomposition was applied and, three minutes thereafter, the clear topcoat composition was immediately applied. The film thicknesses were 15to 20 μm and 35 to 45 μm, respectively, after drying. Then, after 10minutes of standing at room temperature, baking was carried out at 140°C. for30 minutes. In recoatability testing, the baking was carried outat 160° C. for 30 minutes and the recoat layer was baked at 120° C. for30 minutes. Typical performance characteristics and theappearance ofeach coating film thus obtained are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                    Example                                                                       1  2  3   4  5  6  7  8  9  10 11 12 13 14 15                 __________________________________________________________________________    Pigmented base coat composition                                                               1  1  1   2  2  2  3  3  3  4  4  4  5  5  5                  Clear top coat composition                                                                    1  2  3   1  2  3  1  2  3  1  2  3  1  2  3                  Appearance (*1) A  A  A   A  A  A  A  A  A  A  A  A  A  A  A                  Image clarity (*2)                                                                            81 81 79  81 81 79 86 86 85 86 86 85 78 78 76                 Metallic tone (*3)                                                                            A  A  A   A  A  A  A  A  A  A  A  A  A  A  A                  Water resistance (*4)                                                                         A  A  A   A  A  A  A  A  A  A  A  A  A  A  A                  Recoatability (*5)                                                                            A  A  A   A  A  A  A  A  A  A  A  A  A  A  A                  Solvent resistance (*6)                                                       Coating film condition                                                                        B  B  B            A  A  A           B  B  B                  Coating film hardness                                                                         B/H                                                                              B/H                                                                              2B/H         F/H                                                                              F/H                                                                              B/H         B/H                                                                              B/H                                                                              2B/H               __________________________________________________________________________                    Example                                                                       16 17 18 19 20 21 22  23 24 25 26  27 28 29 30                __________________________________________________________________________    Pigmented base coat composition                                                               6  6  6  7  7  7  8   8  8  9  9   9  10 10 10                Clear top coat composition                                                                    1  2  3  1  2  3  1   2  3  1  2   3  1  2  3                 Appearance (*1) A  A  A  A  A  A  A   A  A  A  A   A  A  A  A                 Image clarity (*2)                                                                            78 78 77 78 78 76 84  84 83 84 84  83 84 84 83                Metallic tone (*3)                                                                            A  A  A  A  A  A  A   A  A  A  A   A  A  A  A                 Water resistance (*4)                                                                         A  A  A  A  A  A  A   A  A  A  A   A  A  A  A                 Recoatability (*5)                                                                            A  A  A  A  A  A  A   A  A  A  A   A  A  A  A                 Solvent resistance (*6)                                                       Coating film condition            A   A  A                                    Coating film hardness             F/H F/H                                                                              B/H                                  __________________________________________________________________________                                           Comparative Example                                                           1   2   3   4   5   6                  __________________________________________________________________________                           Pigmented base coat composition                                                               11  11  11  12  12  12                                        Clear top coat composition                                                                    1   2   3   1   2   3                                         Appearance (*1) C   C   C   C   C   C                                         Image clarity (*2)                                                                            38  38  38  37  37  37                                        Metallic tone (*3)                                                                            B   B   B   B   B   B                                         Water resistance (*4)                                                                         B   B   B   B   B   B                                         Recoatability (*5)                                                                            C   C   C   C   C   C                                         Solvent resistance (*6)                                                       Coating film condition                                                                        E   E   E   E   E   E                                         Coating film hardness                                                                         5B/H                                                                              6B/H                                                                              6B/H                                                                              5B/H                                                                              5B/H                                                                              5B/H               __________________________________________________________________________    Test methods                                                                  (*1) Appearance: Coating films were examined for ebullition and shrinkage.     A: no abnormalities; B: some abnormalities observable; C: many                abnormalities observable.                                                    (*2) Image clarity: An image clarity meter (ICM; Suga Shikenki Co.) was        used. The numerical values in Table 1 are ICM values which can range from     0 to 100%. A higher value indicates a higher degree of image clarity and      ICM values of not less than 74 are indices of very good image clarity.       (*3) Metallic tone: Coating films were looked at frontways and evaluated       for metallic glitters and white reflection by the eye. A: glitters and        white reflection; B: no glitters and poor white reflection; C: no glitter    and no white reflection.                                                      (*4) Water resistance: Coated sheet specimens were immersed in warm water      maintained at 40° C. for 240 hours and then evaluated for blisters     by the eye. A: no abnormalities; B: some abnormalities; C: many               abnormalities.                                                               (*5) Recoatability: The same base coat composition and clear coat              composition as used in each example or comparative example were reapplied     to the coated surface and baked at 120° C. for 30 minutes. The         coating film obtained was cut crosswise with a cutting knife, an adhesive     cellophane tape was applied to the coated surface and peeled off abruptly    and the adhesion between the first coating film and second coating film        (clear coating film/base coating film) was evaluated. A: no pe eling; B:      slight extent of peeling; C: remarkable extent of peeling.                   (*6) Solvent resistance: Specimens were immersed in Nisseki Silver gasolin    (trademark of Nippon Oil Co.) for 1 day and then examined for the state an    hardness of coating films. State of coating film: A: no abnormalities; B:      very slight extent of shrinkage; C: some extent of shrinkage; D:              shrinkage; E: marked extent of shrinkage. Coating film hardness: Pencil       hardness after immersion/pencil hardness before immersion.               

We claim:
 1. A method of forming a coating film by forming in sequence apigmented base coat and a clear top coat on a substrate followed byfinishing by the two-coat one-bake technique, the method beingcharacterized by using, as a coating composition for pigmented base coatformation, a composition comprising, as essential components thereof,(1)at least one OH-containing resin selected from the group consisting ofan OH-containing polyester resin and an OH-containing vinyl resin, eachhaving a hydroxyl value of about 20 to 200, (2) a methylol amino resinetherified with an alcohol, (3) a polyorganosiloxane which has, on anaverage, at least two groups, per molecule, each selected from the classconsisting of a silanol group and an alkoxysilane group and has a numberaverage molecular weight of at least 1,000 the polyorganosiloxaneoptionally having, on an average, at least one epoxy group per molecule,the polyorganosiloxane being prepared by subjecting to hydrolysiscondensation a silane compound of the general formula

    R.sup.1.sub.x Si(OR.sup.2).sub.4-x                         (I)

wherein R¹ and R² may be the same or different and each is a hydrocarbongroup containing 1 to 13 carbon atoms and x is 1, the epoxy-containingpolyorganosiloxane being prepared by subjecting to hydrolysiscocondensation a silane compound of the general formula (I) wherein x is1, 2 or 3 and an epoxy-containing silane of the general formula ##STR3##wherein R³ and R⁴ may be the same or different and each is a hydrocarbongroup consisting 1 to 13 carbon atoms, Y is 1, 2 or 3, G is a group ofthe formula ##STR4## wherein R⁵ is a bivalent hydrocarbon groupcontaining 1 to 13 carbon atoms and the R⁶ groups may be the same ordifferent and each is a hydrogen atom or a methyl group, (4) a flakymetal powder and/or a mica powder, and (5) an organic solvent,and using,as a coating composition for clear top coat formation, a compositioncomprising, as essential components thereof, (1) a base resin which is(i) an OH-containing resin further containing at least one groupselected from the class consisting of a silanol group and a hydrolyzablegroup bound directly to a silicon atom within the same molecule or (ii)a mixed resin composed of a resin containing at least one group selectedfrom the class consisting of a silanol group and a hydrolyzable groupbound directly to a silicon atom and an OH-containing resin, (2) amethylol amino resin etherified with an alcohol, and (3) an organicsolvent.
 2. A method of forming coating films as claimed in claim 1,wherein, in the pigmented base coat composition, the proportions of theOH-containing resin, methylol amino resin and polyorganosiloxane are 5to 90% by weight, 5 to 50% by weight and 1 to 40% by weight,respectively, based on the total amount (resin solids) of theOH-containing resin, methylol amino resin and polyorganosiloxane.
 3. Amethod of forming coating films as claimed in claim 1, wherein, in theclear top coat composition, the proportion of the methylol amino resinis 2 to 100 parts by weight per 100 parts by weight of the base resin.